Like many Americans, I got four free Covid at-home rapid antigen tests from the U.S. government by mail recently. But how accurate are they?
The tests I got, on Oct. 4, are BinaxNow brand. While the date on the box shows that they expired July 10, 2023, the FDA.gov website says these tests are effective through Feb. 10, 2024. So … it’s all good, right?
First, it’s great that the government is sending free tests. More testing is good for knowing the extent of the current Covid surge, and believe me, there is a surge — definitely here in the New York area, and elsewhere as well.
But are these tests really effective? We have written from the beginning of the pandemic that tests are not actually 100 percent accurate — there are a lot of false negatives (it says you’re negative when you’re not) and false positives (you might be positive though you are no longer infectious, some experts say, so the positive result may suggest that you need to isolate even if you are not infectious). Still, any testing should be good in a pandemic so we can judge the scope of the problem, right? Well, inaccurate testing is not that great, you’ll probably agree.
Most accurate on 4th day
New information is comng out about testing protocols. First, tests are most accurate on the fourth day of having symptoms, a study published recently in the journal Clinical Infectious Diseases found. So people who test as soon as they have symptoms and get a negative result may decide they don’t have Covid — when if they test later, they might get a positive result.
“In a highly immune adult population, median SARS-CoV-2 viral loads peaked around the fourth day of symptoms,” the study said, explaining that some testing will not detect a light viral load. The people in the study had had Covid, or had been vaccinated or both. The tests could find some infections on the second day, it said, but they were more accurate on the fourth day. The sensitivity was 30-60% on Day 1, vs. 90% on Day 4, the study found.
While much of testing now is done with rapid at-home antigen tests, the polymerase chain reaction (PCR) tests, usually done in a lab or outpatient facility or hospital, were long considered more accurate, and the gold standard. PCR tests are now harder to find, given the end of the public health emergency in the spring, and they often require a patient payment.
From the very beginning of the pandemic, we have been reporting that even the PCR tests are not completely accurate. Doctors told us that this is not uncommon: We all want a test to be completely accurate (hey, pregnancy tests are!) but they are not always. Doctors said that the clinical picture — symptoms, patient history of exposure — holds more meaning for them than a test result.
A 2022 study of studies found that PCR results were unreliable in a surprising number of cases, delivering false negative results — meaning that the results were negative, even though the patients were actually positive for Covid. “This systematic review showed that up to 58% of COVID-19 patients may have initial false-negative RT-PCR results, suggesting the need to implement a correct diagnostic strategy to correctly identify suspected cases, thereby reducing false-negative results and decreasing the disease burden among the population,” said the study, published by the National Library of Medicine.
Confusion among patients and clinicians
People have fragmentary and conflicting information on testing, which has been true from the beginning.
Beyond that, some clinicians believe the tests are accurate. A woman Tweeted the other day: “I’m sick sick sick but three home COVID tests confirm it is not COVID. There aren’t any appointments for PCR tests for weeks and the nurse assured me that the home tests are accurate.”
Reflecting the confusion, another person Tweeted: “Experiencing some mild congestion and my throat is slightly sore but my rapid tests are negative but those aren’t always accurate and now I’m stressed. I feel like the expectation is that I go back to work tomorrow but I have zero confidence that I don’t have COVID.”
Another person Tweeted: “Learned to swab nose, cheek, and the back of the throat to get best results, but done correctly, at home tests are still only 80% accurate. Really, if you’re sick and can’t take a better test to make sure, just assume it’s COVID and take precautions for yourself and others.”
Yet another: “Rapid tests aren’t always accurate, but flowflex from target is the best bet for at home. Also helps to do throat before nose. PCR is always best if possible, though not the cheapest if your insurance doesn’t cover. Plus, I’ve known people to not get a positive until CAlso, different tests have different accuracy ratings.
Caltech study
“New research conducted at Caltech suggests that in many cases, rapid tests that use a nasal swab provide false negatives — suggesting that a person is infection-free even though other parts of their respiratory tract are teeming with the virus. The findings, from the lab of Rustem Ismagilov, Ethel Wilson Bowles and Robert Bowles Professor of Chemistry and Chemical Engineering, appear in a paper published in the journal Microbiology Spectrum on June 15,” Caltech reported in a post.
“Researchers in Ismagilov’s lab tracked viral loads in three places in the human body during the course of a COVID-19 infection: the nose, the throat, and the mouth. Because the nose, throat, and mouth are so closely connected, one might expect to see similar virus levels in those locations. That turns out not to be the case.
“‘Generally, we saw that most people have virus first appear in their throat and in saliva, and then, sometimes days later, in their nose,’ says Alexander Viloria Winnett, biology graduate student and study co-author. ‘Actually, each sample type from a single person follows its own distinct rise and fall of viral load, so it makes a big difference which sample type is used for testing.'”
While it was a very small study, “Importantly, 15 of the 17 study participants had high and presumably infectious levels of virus for at least a day prior to getting a positive antigen test.”
What can you do?
We know tests are unreliable for many reasons. What can you do? Here are some suggestions.
- Check the expiration date of your tests on this FDA page. Different tests have different dates and different longevity.
- To order free tests, use this U.S. Postal Service link.
- Make sure you’re doing the swab for the sample right. Many experts suggest that swabbing nose, cheek and throat gives a better chance of getting any viral particles. Also, some suggest putting the swab deep into the nose, adding that shallow swabs might not pick up virus. Others say “follow the instructions on the box.” We tend to think that a bigger sample has a bigger chance of finding virus particles — so we are in the “swab more” group.
- Be aware that the time from exposure or symptoms to the time of testing may mean a difference in the positive or negative results. There is not a lot of great science on this, but there the new studies referenced above suggest that a first-day test is often not as accurate as a fourth-day test.
- If you have symptoms that suggest Covid, act as if you have it by isolating to protect others, even if you test negative — especially early after exposure and developing symptoms.
- Related: Don’t always believe in the test. Since 2020, we have been reporting on how inaccurate the tests can be — both the Rapid Antigen Tests (RAT) and PCR tests. In the early going, and especially now, we are hearing from clinicians that the test is not a sure thing; you need to think of the clinical picture. Does it look like allergies, or like a sinus infection, but you test negative? It might be Covid.
- What if the test has expired? Should you use it anyway? “The F.D.A. advises people to not use a coronavirus test beyond the extended expiration date. The test parts may degrade or break down, so kits used beyond the extended expiration date could give inaccurate test results, the agency said,” USA Today reported.
- Different tests can give different results. “Investing in better tests than rapids if you’re able bc I’m double FlowFlex negative (w/ cheek & throat swab) but double Lucira positive & developing symptoms. Saved me from giving COVID to my friend,” one woman tweeted. Lucira is in bankruptcy but sold its assets to Pfizer. Lucira is one of several molecular tests authorized by the F.D.A.; the molecular test is a different method from the rapid antigen test. More detail about molecular and other tests can be found here at a New York Times Wirecutter rundown of tests; there may be a paywall blocking access to some individuals.
